The Rev. Joseph Musser’s family has always lived in the region of Alsace, but not always in the same country.

His grandfather fought for the Germans in World War I, and his father for the French in World War II. Today, no one is fighting anymore. His great-niece lives in France but works in Germany, crossing the border her ancestors died fighting over without even noticing it.

It is this era of peace and borderless prosperity that champions of the European Union consider the bloc’s singular achievement.

“The foundation of the European Union is the memory of war,” said the Reverend Musser, 72. “But that memory is fading.”

On Sunday, as dozens of world leaders gather in Paris to mark the centenary of the armistice that ended World War I, the chain of memory that binds Mr. Musser’s family — and all of Europe — is growing brittle.

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A migrant camp on the Greek island of Lesbos. Europeans have been shaped by a decade-long financial crisis, an influx of migrants from Africa and the Middle East, and a sense that the promise of a united Europe was not delivering.CreditMauricio Lima for The New York Times

The anniversary comes amid a feeling of gloom and insecurity as the old demons of chauvinism and ethnic division are again spreading across the Continent. And as memory turns into history, one question looms large: Can we learn from history without having lived it ourselves?

In the aftermath of their cataclysmic wars, Europeans banded together in shared determination to subdue the forces of nationalism and ethnic hatred with a vision of a European Union. It is no coincidence that the bloc placed part of its institutional headquarters in Alsace’s capital, Strasbourg.

But today, its younger generations have no memory of industrialized slaughter. Instead, their consciousness has been shaped by a decade-long financial crisis, an influx of migrants from Africa and the Middle East, and a sense that the promise of a united Europe is not delivering. To some it feels that Europe’s bloody last century might as well be the Stone Age.

Yet World War I killed more than 16 million soldiers and civilians, and its legacies continue to shape Europe.

‘‘The war to end all wars’’ set the scene for an even more devastating conflict and the barbarism of genocide. Winston Churchill, Britain’s legendary wartime leader, thought of 1914-1945 as one long war.

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A model of the River Clyde, a British ship that took part in the World War I landings in Gallipoli, Turkey.CreditTomas Munita for The New York Times

“Those who fail to learn from history are condemned to repeat it,” he said in 1948.

Chancellor Angela Merkel of Germany, whose decision to welcome more than a million migrants to Germany in 2015 first became a symbol of a liberal European order, then a rallying cry for a resurgent far-right, said the jury is still out on whether Europe will heed the lessons of its past.

“We now live in a time in which the eyewitnesses of this terrible period of German history are dying,” she said of World War II. “In this phase, it will be decided whether we have really learned from history.”

Indeed, the last World War I veteran died in 2012. And the number of those who experienced World War II and the Holocaust is rapidly shrinking, too.

Politicians are apt to use history selectively when it suits them. But the history in this case is ominous.

Now as then, Europe’s political center is weak and the fringes are radicalizing. Nationalism, laced with ethnic hatred, has been gaining momentum. Populists sit in several European governments.

In Italy, a founding member of the European Union, Matteo Salvini, the nationalist deputy prime minister, has turned away migrant boats and called for the expulsion of Roma. Prime Minister Viktor Orban of Hungary speaks of a “Muslim takeover” and unapologetically flaunts his version of “illiberal democracy.”

“In 1990, Europe was our future,” he said earlier this year. “Now, we are Europe’s future.”

The political discourse is deteriorating in familiar ways, too. In Germany, the far right has become the main voice of opposition in Parliament, mocking the mainstream media as “Lügenpresse,” or lying press — a term that was first used by the Nazis in the 1920s before their ascent to power.

Traute Lafrenz, the last surviving member of the White Rose, an anti-Hitler student resistance group in the 1940s, said she got goose bumps seeing images of Hitler salutes at far-right riots in the eastern German city of Chemnitz recently.

“Maybe it’s no coincidence,” Ms. Lafrenz, now 99, told Der Spiegel. “We are dying out and at the same time everything is coming back again.”

After World War II, the European Union sought to prevent anything like it from happening again by gradually creating a common market, a common currency, a passport-free travel zone and by pooling sovereignty in a number of areas.

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A photograph of the main square in Ypres, Belgium, during World War I is featured in a restaurant in the city. Half a million men or more died or were wounded there from July 31 to Nov. 10, 1917.CreditDmitry Kostyukov for The New York Times

But on Sunday, standing next to Ms. Merkel and her host, the fiercely pro-European French president, Emmanuel Macron, will be a number of nationalist leaders who would like nothing more than to pull the European Union apart — among them President Trump, President Vladimir V. Putin of Russia and President Recep Tayyip Erdogan of Turkey.

Historians guard against drawing direct parallels between the fragile aftermath of World War I and the present, pointing to a number of notable differences.

Before World War I, a Europe of empires had just become a Europe of nation states; there was no tried and tested tradition of liberal democracy. Economic hardship was on another level altogether; children were dying of malnutrition in Berlin.

Above all, there is not now the kind of militaristic culture that was utterly mainstream in Europe at the time. France and Germany, archenemies for centuries, are closely allied.

“What is being eroded today, is being eroded from a much higher level than anything we had ever achieved in Europe in the past,” said Timothy Garton Ash, professor of European history at the University of Oxford.

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The fields of Verdun, France. The Battle of Verdun, fought from Feb. 21 to Dec. 18, 1916, was the longest single battle of World War I.CreditTomas Munita for The New York Times

Still, Mr. Garton Ash sees 1918 as a warning that democracy and peace can never be taken for granted.

“It’s a really sobering reminder that what seems like some sort of eternal order can very rapidly collapse,” he said.

In that sense, if Europe’s motto after World War II was “never again,” the lesson of World War I is “it could happen again.”

Daniel Schönpflug, a German historian who recently published “A World on Edge,” an evocative book tracking 22 characters in the interwar period, points out that for centuries, periods of prolonged war in Europe’s violent history have been followed by periods of prolonged peace.

“But once the generation with living memory of fighting had died, the next war came along,” Mr. Schönpflug said. “History teaches us that when the generation that experienced war dies out, caution diminishes and naiveté toward war increases.”

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The annual Eichsfeld Day festival, a gathering of the National Democratic Party, a group of avowed neo-Nazis. Chancellor Angela Merkel of Germany has acknowledged the jury is still out on whether Europe will heed the lessons of its past.CreditMauricio Lima for The New York Times

“That means we have to be very careful today,” he said.

In 1918, the artist Paul Klee made ‘‘The Comet of Paris,’’ a tightrope walker hovering precariously in the air with a comet searing through the sky above and the Eiffel Tower below. What is unnerving about the image is that one cannot discern the rope even though one knows it is there.

“It sums up where people were then, and in a way where we are today,” said Mr. Schönpflug.

No one knows what might come next. Europe has entered the unknown.

In 1929, as it happened, people entered a murderous decade without even knowing.

“That’s what’s so eerie looking back,” said James Hawes, a historian and author of The Shortest History of Germany. “Right up to 1931-32, no one realized what was about to happen. They thought they were just entering another decade.”

What might future historians write about the Europe of 2018?

Antony Beevor, author of a numerous best-selling history books, is pessimistic. The moral dilemmas of the future will undo European liberal democracy, he predicts. The migration crisis of 2015 was only a foretaste of what is to come.

“Future waves of migration are inevitable and Europe is their main destination,” Mr. Beevor said, pointing to the disruptive forces of poverty and climate change in developing countries as the main reasons.

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A mural honoring Gavrilo Princip, the Serbian nationalist who killed Archduke Franz Ferdinand and his wife, Sophie, setting off World War I. It is featured in Andricgrad, a model village in Visegrad, Bosnia.CreditAndrew Testa for The New York Times

“European leaders will face the choice of turning back starving refugees or of handing ammunition to the far right and eroding the fabric in their own societies,” he said.

Others see it differently. Niall Ferguson, a senior fellow at the Hoover Institution, says the biggest problem facing Europe is not populism but the incomplete currency union of the euro.

“The major threat of Europe at the moment is not Orban or Salvini, the major threat is that the E.U.’s institutional arrangement is unstable,” Mr. Ferguson said. Mr. Macron’s ambition has been to fix that; but there is no consensus backing him.

Whatever the future of Europe’s institutions, one big difference from 100 years ago is that the Continent is no longer at the heart geopolitics.

"A century ago, Europe was the center of the world — even if it was the dark tragic center of the world,” said Dominique Moïsi, a French author and thinker. “Today we might be back to tragedy but not to centrality.”

“History is moving elsewhere,” he said.

That, too, should be a motivation to shore up European integration, says the Reverend Musser in Alsace. One of his grandnieces is doing an internship in China, not Europe.

Bones, bombs and bullets remain in the soil of Alsace, a region switched back and forth between various incarnations of France and Germany five times between the Thirty Years’ War that ended in 1648 and the devastation of World War II.

Local residents joke that Alsatians still keep German street signs in their basements, just in case.

The Reverend Musser puts it this way: “Alsace is a reminder of how much has been won in Europe — and how much can be lost.”

Christopher F. Schuetze contributed reporting from Berlin.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Century Later, War’s Demons Revisit Europe. Order Reprints | Today’s Paper | Subscribe